Devices for executing such methods are categorized under the term Laser-Doppler Anemometer (called hereinafter LD anemometers for simplicity's sake) which operate on the basis of two entirely different methods for determining the speed of particle flows.
a. In a single beam LD anemometer (also referred to as heterodyne or homodyne or reference beam LD anemometer) the Doppler shift of a scattered light source is used to determine the speed. The radiation of a coherent light source is directed to the volume to be measured and the light scattered by the particle stream is Doppler-shifted. The Doppler-shifted scattered wave is impressed on a detector over a non-scattered, coherent light beam; the electrical signal being generated in the course of this superimposition process indicates the difference frequency between scattered and non-scattered light sources.
If the reference radiation has the same frequency as the radiation directed on the volume to be measured, the difference frequency on the detector equals the amount of the Doppler shift; this is called a homodyne LD anemometer.
If the frequency of the reference radiation is shifted in respect to the frequency of the radiation directed on the volume to be measured, then this is a heterodyne LD anemometer. To be able to determine the sign of the velocity vector in a measuring process, the amount of the frequency shift of the reference beam must be larger than or at least equal to the Doppler shift of the scattered wave. This prerequisite has been met in all heterodyne LD anemometers known to-date.
b. In contrast to the single beam LD anemometer, in the two-beam-crossed-beam or the so-called crossed beam method the radiation from two coherent light sources is crossed at a defined angle in the volume to be measured, because of which an interference pattern forms in the volume to be measured. A particle moving through this interference pattern creates a periodic change of the scattered light; this appears light in the interference maximums and dark in the minimums.
It is always possible to definitely assign LD anemometers to single- or multi-beam systems based on the difference of the measuring techniques sketched under a and b above. Accordingly, the present method is a single beam system in the form of a heterodyne or reference beam LD method, clearly shown in the preamble of the main claim.
The velocity of the scattering object is determined in the Doppler-laser method from the Doppler shift of a light wave scattered by a moving object. However, with the customary homodyne method the amount of the velocity component can be measured only in the direction of observation. The sign of this velocity component can only be determined by means of a heterodyne method, as will be explained in detail below.
The coherent superimposition on a photo-electrical element of a light wave with a frequency f.sub.g on a non-scattered light source with a frequency of f.sub.r creates a current or voltage signal with a frequency of .DELTA.f.sub.m, the size of which equals the amount of the difference of the frequencies f.sub.g and f.sub.r : EQU .DELTA.f.sub.m =.vertline.f.sub.g -f.sub.r .vertline. (1)
If the same coherent light source with the frequency f.sub.o is made the basis of both light waves, only the frequency change .DELTA.f.sub.D (Doppler shift) caused by a scattering action becomes relevant, so that for the frequencies f.sub.r and f.sub.g of the non-scattered and the scattered light waves and thus for the difference frequency .DELTA.f.sub.m the following applies: EQU f.sub.r =f.sub.o (2) EQU f.sub.g =f.sub.o +.DELTA.f.sub.D (3) EQU .DELTA.f.sub.m =.vertline.f.sub.r -f.sub.g .vertline.=.vertline..DELTA.f.sub.D .vertline. ( 4)
In this homodyne case the measured difference frequency .DELTA.f.sub.m equals the amount of the Doppler shift .DELTA.f.sub.D.
The homodyne LD method is an active measuring method used, for example, to determine wind direction; a continuous signal laser is used as beam source, the output of which is focused by means of a telescope on a measuring volume at the distance R (FIG. 1). Additionally, a wind vector w is shown in FIG. 1, the azimuth angle .phi.=0 was selected in FIG. 1, i.e. the wind vector w lies in the drawing plane.
Part of the beam is scattered back from aerosol particles moving through the volume to be measured with the velocity of a stream of air. The scattered beam experiences a frequency change .DELTA.f.sub.D because of a Doppler shift of ##EQU1## where f.sub.o is the frequency of the laser, c is the speed of light and V.sub.LOS is the component of the wind speed in the direction of measuring, i.e. the radial or line-of-sight component.
A signal (in the form of the light scattered back) with a frequency (f.sub.o +.DELTA.f.sub.D) and a part of the emitted laser light as reference signal with the reference f.sub.o are impressed on a detector (heterodyne reception) and there generate an electrical signal, the AC portion of which contains the differential frequency .DELTA.f.sub.m between reference beam and signal beam.
The method of the so-called conical scan is used to determine wind velocity and wind speed; here the scanning beam is pivoted below a fixed elevation angle .theta. respect to the vertical and the vertical is pivoted as axis by an azimuth angle .phi.. Thus a wind zone is being scanned at an altitude H along the envelope of a cone on the basic circle of an observation cone having the radius r at the distance R (see FIG. 1).
In a homogenous wind zone the result for the observable wind component V.sub.LOS as a function of the azimuth angle .phi. is a sinus function (FIG. 2): EQU V.sub.LOS (.phi.)=A sin ( .phi.+.phi..sub.O)+D (6)
with an amplitude A, a displacement D and an initial phase .phi..sub.o. The result for the wind component V.sub.LOS in the direction of observation as a function of the azimuth angle .phi. is
a. if the vertical wind component is missing, no displacement, i.e. D=0,
b. with a vertical wind component upward, a displacement of D&gt;0, and
c. with a vertical wind component downwards, a displacement D&lt;0.
All magnitudes of a wind zone can be determined from the path of this function, i.e. the horizontal wind velocity from ##EQU2## and the vertical wind velocity from ##EQU3##
The wind direction is shown by .phi. .sub.min (i.e. the wind veotor points in the direction .phi. .sub.min). The value .phi. .sub.o determines the orientation of the measuring axis in relation to the absolute geographical direction. Because the measuring axis can be optionally oriented, .phi. .sub.o =0 has been assumed below, as was the case already in FIGS. 1 and 2.
As explained above, in particular in connection with equations (3) and (4), in the homodyne method the amount of the frequency shift .DELTA.f.sub.D is measured on the basis of the Doppler effect. The measured value is a (positive) frequency and is: EQU .DELTA.f.sub.m =.vertline..DELTA.f.sub.D .vertline..about..vertline.V.sub.LOS .vertline. (8)
The result of the described and known method of the so-called conical scan thus is: ##EQU4##
Therefore the amount of a displaced sinus function is being measured. It has been sketched in detail in FIG. 3 for the cases already mentioned above, namely
a. D=0 (no vertical wind); FIG. 3a; PA1 b. D&gt;0 (vertical wind upward); FIG. 3b; PA1 c. D&lt;0 (vertical wind downward); FIG. 3c. PA1 in case a (i.e. for .DELTA.f.sub.s &gt;0) negative, and PA1 in case b (i.e. for .DELTA.f.sub.s &lt;0) positive. PA1 in case a (i.e. for .DELTA.f.sub.s &gt;0) positive, and PA1 in case b (i.e. for .DELTA.f.sub.s &gt;0) negative. PA1 Case a 1 corresponds to case c with .DELTA.f.sub.s (t.sub.1)=0, and PA1 Case a 2 corresponds to case b with .DELTA.f.sub.s (t.sub.1)=0.
As can be clearly seen from this, a differentiation between the absolute minimum and the absolute maximum of the function is basically no longer possible. It is therefore only possible to exactly determine the wind direction to .+-.180.degree.; however, this means that the definition of the sign of the horizontal wind component is not possible with the homodyne method; for the same reasons a determination of the sign of the vertical wind direction is also impossible.
The indefiniteness of direction in the homodyne method can be removed by means of an additional (positive) frequency shift .DELTA.f.sub.s between signal and reference beam. This basic idea of the known heterodyne methods for the determination of the sign is contained, for example, in British Patent 1 554 561 (1975), in U.S. Pat. No. 3,428,816 (1969), in German Patent DE-PS 34 40 376 or in German Published, Non-examined Patent Application DE-OS 37 13 229.
If in a reference light wave a frequency other than f.sub.l is chosen as the frequency f.sub.o of the light wave prior to the scattering action, then the following is true: EQU f.sub.r =f.sub.o +.DELTA.f.sub.s =f.sub.l (10) EQU f.sub.g =f.sub.o +.DELTA.f.sub.D (11)
the difference of the frequencies f.sub.o and f.sub.l being expressed by .DELTA.f.sub.s. The reference frequency f.sub.l can be generated from the frequency f.sub.o by a frequency shift .DELTA.f.sub.s. In this heterodyne case the following is true for the measured difference frequency: EQU f.sub.m =.vertline.f.sub.r -f.sub.g .vertline.=.vertline..DELTA.f.sub.s -.DELTA.f.sub.D .vertline. (12)
Together with the requirement customary in known heterodyne systems that the frequency shift .DELTA.f.sub.s of the non-scattered radiation be larger than or equal to in its amount to the amount of the Doppler shift f.sub.D, the determination of the sign of .DELTA.f.sub.D can be obtained from a measurement of .DELTA.f.sub.m. The following cases can then be differentiated:
Case a: .DELTA.f.sub.s &gt;0
In this case the amount lines may be omitted and the following is true for the difference frequency .DELTA.f.sub.m : ##EQU5##
Case b: .DELTA.f.sub.s &lt;0
In this case the signs are reversed if the amount lines are omitted and the following is true for the difference frequency .DELTA.f.sub.m : ##EQU6##
In both cases the frequency shift f.sub.m of the measured signal in relation to the known frequency shift of the reference beam .vertline..DELTA.f.sub.s .vertline. is examined:
If the measured frequency shift .DELTA.f.sub.m is larger than .vertline..DELTA.f.sub.s .vertline., the sign of the Doppler shift .DELTA.f.sub.D is
If the difference frequency .DELTA.f.sub.m is smaller than the amount of the difference shift .vertline..DELTA.f.sub.2 .vertline., the sign of the Doppler shift .DELTA.f.sub.D is
Thus, one measuring operation is sufficient to determine the sign of the Doppler shift, provided that the amount of the frequency shift .DELTA.f.sub.s of the reference radiation is larger than or equal to the amount of the Doppler shift .DELTA.F.sub.D.
The advantage of being able to determine the sign of the Doppler shift .DELTA.f.sub.D in one single measuring operation is, however, offset to a large degree by serious technical problems in generating a sufficiently large frequency shift .DELTA.f.sub.s.
In all heterodyne methods for the determination of the sign so far known, the frequency shift .DELTA.f.sub.s must be large compared to the Doppler shift .DELTA.f.sub.D to be measured, i.e. .DELTA.f.sub.s &gt;&gt;.DELTA.f.sub.D. As can be seen from FIG. 4, it is then possible to determine the sign of the Doppler shift ( .DELTA.f.sub.D) relative to the frequency shift .DELTA.f.sub.s and thus the sign of the wind vector.
However, the applicability of the known heterodyne methods is highly restricted or the known heterodyne methods are hardly used in actuality, mainly because of the high degree of technical effort required, such as additional optical components which are hard to adjust and sensitive, additional lasers, etc. The heterodyne method has had wider application only in airborne systems where a large additional Doppler displacement is a priori present because of the proper motion.